CN103454637B - Terahertz inverse synthetic aperture radar imaging method based on frequency modulation step frequency - Google Patents

Abstract

The invention discloses a terahertz inverse synthetic aperture radar imaging method based on a frequency modulation step frequency to solve the problem that in the prior art, the range resolution and the direction resolution of inverse synthetic aperture radar imaging in the microwave band are not high. The method includes the steps of firstly, obtaining a terahertz pulse echo signal; secondly, conducting pulse compression; thirdly, synthesizing a broadband; fourthly, conducting windowing and correcting; fifthly, conducting distance Doppler imaging; sixthly, obtaining an imaged picture. According to the method, the characteristic that the terahertz wave has the high frequency and the broadband is combined with the frequency modulation step frequency system so that the high-resolution terahertz inverse synthetic aperture radar imaging can be achieved through the method of synthesizing the broadband.

Description

Based on the Terahertz inverse synthetic aperture radar imaging method of frequency modulation Step Frequency

Technical field

The invention belongs to communication technical field, further relate to a kind of Terahertz inverse synthetic aperture radar imaging method based on frequency modulation Step Frequency in inverse synthetic aperture radar (ISAR) technical field.The present invention adopts the signal of frequency modulation Step Frequency system and terahertz wave band to realize high-resolution inverse synthetic aperture radar imaging, to solve the not high problem of conventional microwave band signal inverse synthetic aperture radar imaging resolution, obtain high-resolution inverse synthetic aperture radar imaging figure.

Background technology

THz wave is the electromagnetic wave (frequency is 0.1THz to 10THz) between millimeter wave and infrared light, is also the frequency range that last mankind is not yet completely cognitive and utilize.THz wave has merged the advantage of microwave and millimeter wave and infrared light, be adapted to the characteristic of moderate beam angle, wide system bandwidth and large Doppler shift especially, be more suitable for the realization of the extremely narrow antenna beam of significant signal bandwidth sum, be more conducive to high-resolution inverse synthetic aperture radar imaging.

Relative to the radar of conventional microwave wave band, Terahertz radar with its High Range Resolution, super large signal bandwidth, strong penetrating power, low intercepting and capturing rate, common-path interference, superior anti-stealthy and penetrate plasma capable and have powerful technical advantage to realize numerous functions of the system such as radar detection and imaging, but also brings huge challenge to imaging algorithm due to himself feature.Frequency modulation stepped frequency signal adopts linear FM signal as the subpulse of stepped frequency signal, there is the feature of chirp and Step Frequency pulse, therefore while acquisition High Range Resolution, the requirement to digital signal processor instant bandwidth can be reduced, the data transfer rate of raising system, ensure the operating distance of signal simultaneously, realize Terahertz inverse synthetic aperture radar (ISAR) carries out high-resolution imaging object to target, therefore the Terahertz radar of frequency modulation Step Frequency system has very important application prospect at high-resolution inverse synthetic aperture radar imaging.

A kind of data processing method of THZ level large bandwidth laser synthetic aperture radar image-forming system is disclosed in the patented claim " data processing method of THZ level large bandwidth laser synthetic aperture radar image-forming system " (application number: 201210091702.6, publication number: CN102636776A) that Shanghai Institute of Technical Physics of the Chinese Academy of Sciences proposes.First the method adopts based on Linear Tuning laser pulse signal on a large scale, adopts homodyne coherent detection technology, balance detection technology and synthetic aperture technique, finally utilizes special data processor, calculate the two dimensional image of target.The weak point of the method is: adopt laser pulse signal, and the impact being vulnerable to weather and air during work makes laser beam that distortion and shake occur, and directly affects the operating accuracy of radar, can not reach the high-resolution effect of inverse synthetic aperture radar imaging.

A kind of two-dimentional terahertz imaging system and formation method thereof is disclosed in the patented claim " a kind of two-dimentional terahertz imaging system and formation method thereof " (application number: 201210108140.1, publication number: CN102621070A) that Wu Zhouling proposes.Imaging object is positioned between terahertz imaging lens and Terahertz condenser lens when utilizing two-dimentional terahertz imaging system to obtain image by the method, then the laser initiation terahertz light lead antenna array sent by femto-second laser produces terahertz wave beam group, converge in terahertz wave detector by Terahertz condenser lens again, obtain two-dimentional Terahertz image.The weak point of the method is: the method is based on optoelectronic formation method, can only carry out imaging to in-plant object, and the resolution that imaging reaches does not utilize the resolution of the method imaging of inverse synthetic aperture high.

Summary of the invention

The present invention is directed to the problem that in above-mentioned prior art, inverse synthetic aperture radar imaging resolution is not high, propose a kind of based on the Terahertz inverse synthetic aperture radar imaging method based on frequency modulation Step Frequency, by synthesis large bandwidth, high-frequency signal, the resolution of inverse synthetic aperture radar imaging is improved.

Realize concrete steps of the present invention as follows:

(1) terahertz pulse echoed signal is obtained:

The Terahertz frequency modulation stepped frequency signal of Terahertz radar receiver receiving target scattering point reflection, obtain Terahertz frequency modulation Step Frequency echoed signal, Terahertz frequency modulation Step Frequency echoed signal is made up of Terahertz frequency modulation Step Frequency train of impulses, and every group pulse string is made up of chirp.

(2) pulse compression:

Pulse compression is carried out to each Terahertz frequency modulation Step Frequency echoed signal, obtains the signal after pulse compression.

(3) large bandwidth is synthesized:

3a) frequency displacement is carried out to the signal after each pulse compression often organized in Terahertz frequency modulation Step Frequency train of impulses, obtain the pulse signal after frequency displacement;

3b) pulse signal after frequency displacement is synthesized a large bandwidth signal according to the following formula:

$X\left(t\right)=\underset{m=1}{\overset{M}{\mathrm{\Σ}}}{y}_m\left(t\right)$

Wherein, X (t) represents the large bandwidth signal after synthesis, t represents the time that radar receiver reception chirp experiences, M represents the total number of chirp often organized Terahertz frequency modulation Step Frequency train of impulses and comprise, M value be more than or equal to 1 positive integer, m represents m chirp, and m span is the positive integer of 1 ~ M, y _mt () represents the pulse signal after the signal frequency shift after m pulse compression.

(4) windowing correction:

According to the following formula, to the large bandwidth signal windowing correction after synthesis, the revised large bandwidth signal of windowing is obtained:

Z(t)=IFFT[FFT[X(t)]×H(f)]

Wherein, Z (t) represents the revised large bandwidth signal of windowing, t represents the time that radar receiver reception chirp experiences, IFFT [] expression carries out inverse Fourier transform to the large bandwidth signal of windowing correction, FFT [] represents that the large bandwidth signal after to synthesis carries out Fourier transform, X (t) represents the large bandwidth signal after synthesis, and H (f) represents the rectangular window of frequency domain.

(5) RANGE-DOPPLER IMAGING:

5a) to the orientation of the revised large bandwidth signal of windowing to carrying out Fourier transform, obtain Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture;

5b) to the distance of the revised large bandwidth signal of windowing to carrying out pulse compression, obtain Terahertz inverse synthetic aperture radar (ISAR) target scattering point distance to full resolution pricture.

(6) image is obtained:

By Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture and distance to full resolution pricture comprehensively in two-dimensional coordinate system, obtain two-dimentional high-resolution Terahertz inverse synthetic aperture radar imaging figure.

The present invention has the following advantages compared with prior art:

The first, the present invention, by selecting THz wave as the signal of inverse synthetic aperture radar imaging, overcomes the problem that in prior art, inverse synthetic aperture radar imaging azimuth resolution is not high, achieves the orientation high-resolution in inverse synthetic aperture radar imaging.

Second, frequency modulation stepped frequency signal system is adopted in the present invention, large bandwidth signal is obtained by the mode that anamorphic zone is roomy, overcome the problem that the little range resolution caused of inverse synthetic aperture radar imaging bandwidth in prior art is not high, achieve distance in inverse synthetic aperture radar imaging to high-resolution.

Accompanying drawing explanation

Fig. 1 is process flow diagram of the present invention;

Fig. 2 is the model schematic of target scattering point in the present invention;

Fig. 3 is the inverse synthetic aperture radar imaging figure of target scattering point in the present invention.

Embodiment

Below in conjunction with accompanying drawing, the present invention is described in further detail.

With reference to accompanying drawing 1, concrete steps of the present invention are as follows:

Step 1, obtains terahertz pulse echoed signal.

The Terahertz frequency modulation stepped frequency signal of Terahertz radar receiver receiving target scattering point reflection, obtain Terahertz frequency modulation Step Frequency echoed signal, Terahertz frequency modulation Step Frequency echoed signal is made up of Terahertz frequency modulation Step Frequency train of impulses, and every group pulse string is made up of chirp.

Utilize the mathematical model of linear FM signal to obtain the mathematical model of frequency modulation Step Frequency, mathematical model used is as follows: one group of Terahertz frequency modulation stepped frequency signal is made up of M chirp, and m the chirp wherein launched can be written as:

$c_m\left(t\right)=\mathrm{rect}\left(\frac{t}{T}\right)e^{({\mathrm{j\πkt}}^2+{j2\mathrm{\πf}}_m)}$

Wherein, c _mt () represents m Terahertz chirp pulse signal, m value is the positive integer of 1 ~ M, M represents the total number of chirp that every group pulse string comprises, M value be more than or equal to 1 positive integer, t represents the time that radar receiver reception chirp experiences, and rect () represents time domain rectangle window, and T represents the cycle of chirp, k represents the frequency modulation rate of chirp, f _mrepresent the carrier frequency of m Terahertz chirp.

With reference to accompanying drawing 2, set up the coordinate system of radar and target scattering point, R ₀represent the distance of target scattering dot center 0 to radar, the coordinate of scattering point P under x '-y ' coordinate system is (x ₀, y ₀), under x-y coordinate system (rotating coordinate system), coordinate is (x, y), and true origin is R to the distance of radar ₀, then:

x==rcos(θ+α)=x ₀cosθ-y ₀sinθ

y==rsin(θ+α)=x ₀sinθ+y ₀cosθ

Wherein, x represents the horizontal ordinate of scattering point P under x-y coordinate system (rotating coordinate system), and y represents the ordinate of scattering point P under x-y coordinate system (rotating coordinate system), x ₀represent the horizontal ordinate of scattering point P under coordinate system x '-y ', y ₀represent the ordinate of scattering point P under coordinate system x '-y ', α represents the angle between target scattering point and the horizontal ordinate of rotating coordinate system x-y, and θ represents the angle between the horizontal ordinate of target scattering point and rotating coordinate system x '-y '.

Then centered by radar, set up the coordinate system parallel with x '-y ' coordinate system, now scattering point coordinate is (x+R ₀, y), then target scattering point to the distance of radar is $R=\sqrt{{(x+R_0)}^2+y^2}\≈R_0+x_0\cos \mathrm{\θ}-y_0\sin \mathrm{\θ}.$ Only consider that target scattering point rotates platform center rotating, the angle that when launching the i-th group pulse string, the relative radar of target turns over is θ _i=ω × i × PRT, i=0,1 ..., N _a-1, N _arepresent total group of number of Terahertz frequency modulation Step Frequency train of impulses, can obtain target scattering point coordinate in a coordinate system, thus to try to achieve target to the distance of radar be R (n), wherein n represents the n-th scattering point.

According to following formula, obtaining Terahertz chirp echoes signal is:

$s_r\left(t\right)=\underset{n=1}{\overset{N}{\mathrm{\Σ}}}{c}_m(t-\frac{2R\left(n\right)}{c})$

Wherein, s _rt () represents m Terahertz chirp echoes signal, t represents the time that radar receiver reception chirp experiences, and n represents the n-th target scattering point, and N represents target scattering point sum, c _mt () represents m Terahertz chirp pulse signal, R (n) represents the distance of the n-th target scattering point to radar, and c represents the light velocity.

Step 2, pulse compression.

Pulse compression is carried out to each Terahertz frequency modulation Step Frequency echoed signal, obtains the signal after pulse compression.

Step 3, synthesis large bandwidth.

The first step, carries out frequency displacement to the signal after each pulse compression often organized in Terahertz frequency modulation Step Frequency train of impulses, obtains the pulse signal after frequency displacement.Described frequency displacement realizes according to the following formula:

$y_m\left(t\right)=s_m\left(t\right)e^{j2\mathrm{\π\Δ}{f}_{m}t}$

Wherein, y _mt () represents the pulse signal after m frequency displacement, m represents m chirp, m span is the positive integer of 1 ~ M, M represents the total number of chirp often organized Terahertz frequency modulation Step Frequency train of impulses and comprise, M value be more than or equal to 1 positive integer, t represents that radar receiver receives time of experiencing of chirp; s _mt () represents the signal after m pulse compression, Δ f _mrepresent the frequency shift amount of the signal after m pulse compression.

Second step, synthesizes a large bandwidth signal according to the following formula by the pulse signal after frequency displacement:

$X\left(t\right)=\underset{m=1}{\overset{M}{\mathrm{\Σ}}}{y}_m\left(t\right)$

Wherein, X (t) represents the large bandwidth signal after synthesis, t represents the time that radar receiver reception chirp experiences, M represents the total number of chirp often organized Terahertz frequency modulation Step Frequency train of impulses and comprise, M value be more than or equal to 1 positive integer, m represents m chirp, and m span is the positive integer of 1 ~ M, y _mt () represents the pulse signal after the signal frequency shift after m pulse compression.

Step 4, windowing correction.

According to the following formula, to the large bandwidth signal windowing correction after synthesis, the revised large bandwidth signal of windowing is obtained:

Z(t)=IFFT[FFT[X(t)]×H(f)]

Wherein, Z (t) represents the revised large bandwidth signal of windowing, t represents the time that radar receiver reception chirp experiences, IFFT [] expression carries out inverse Fourier transform to the large bandwidth signal of windowing correction, FFT [] represents that the large bandwidth signal after to synthesis carries out Fourier transform, X (t) represents the large bandwidth signal after synthesis, H (f) represents the rectangular window of frequency domain, through the signal that windowing revised large bandwidth signal is fairly perfect, filter out the burr of corresponding clutter and signal generation.

Step 5, RANGE-DOPPLER IMAGING.

By the orientation of the large bandwidth signal of synthesis to carrying out FFT process, obtain orientation to full resolution pricture; By the distance of the large bandwidth signal of synthesis to carrying out process of pulse-compression, obtain distance to full resolution pricture.

Step 6, obtains image.

By Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture and distance to full resolution pricture comprehensively in two-dimensional coordinate system, obtain two-dimentional high-resolution Terahertz inverse synthetic aperture radar imaging figure.

Effect of the present invention can be further illustrated by following emulation.

1. simulated conditions:

Emulation of the present invention carries out under the software environment of MATLAB R2012a, and the translation of target scattering point compensates, only considers that a target rotates platform center rotating.

2. emulate content:

The parameter of Radar Target Scatter point: 2 target scattering points are respectively R to the distance of radar ₁=2498m and R ₂=2501m, target scattering dot center is to the distance R of radar ₀=2500m, target rotation angle θ=1 ⁰, rotating speed of target ω=0.04rad/s.The signal parameter launched: linear frequency modulation rate K _r=5 × 10 ¹³, the burst blocks number launching Terahertz frequency modulation stepped frequency signal is N _a=256, often a stringly comprise M _r=10 subpulses.The parameter of radar ISAR imaging system: range resolution is azimuth resolution is light velocity c=3 × 10 ⁸m/s.

Inverse synthetic aperture radar (ISAR) receives two kinds of signals: the first is the centre frequency 120GHz of frequency modulation stepped frequency signal, signal total bandwidth 5GHz, and Equations of The Second Kind is the centre frequency 30GHz of linear FM signal, signal bandwidth 1GHz, compares simulation result.

3. analysis of simulation result:

With reference to accompanying drawing 3, analysis is compared to simulation imaging result of the present invention, wherein, accompanying drawing 3(a) be that prior art adopts linear FM signal, linear FM signal centre frequency is 30GHz, and bandwidth is the imaging effect figure that 1GHz does not adopt bandwidth to synthesize.Accompanying drawing 3(b) be the imaging effect figure of the inventive method, adopt the frequency modulation stepped frequency signal of Terahertz, frequency modulation stepped frequency signal centre frequency is 120GHz, and bandwidth is the imaging effect figure that 5GHz adopts synthesis large bandwidth.

Contrast accompanying drawing 3(a) and accompanying drawing 3(b), the resolution of the inverse synthetic aperture radar imaging figure that the inventive method obtains with the signal of synthesis large bandwidth due to employing high-frequency, comprise range resolution and azimuthal resolution, the imaging resolution of all little than bandwidth traditional microwave wave band is high, thus is demonstrated correctness and the validity of the inventive method by Fig. 3 (b) imaging effect.

Claims (2)

1., based on a Terahertz inverse synthetic aperture radar imaging method for frequency modulation Step Frequency, comprise the steps:

(1) terahertz pulse echoed signal is obtained:

The Terahertz frequency modulation stepped frequency signal of Terahertz radar receiver receiving target scattering point reflection, obtain Terahertz frequency modulation Step Frequency echoed signal, Terahertz frequency modulation Step Frequency echoed signal is made up of Terahertz frequency modulation Step Frequency train of impulses, and every group pulse string is made up of chirp;

(2) pulse compression:

Pulse compression is carried out to each Terahertz frequency modulation Step Frequency echoed signal, obtains the signal after pulse compression;

(3) large bandwidth is synthesized:

3a) frequency displacement is carried out to the signal after each pulse compression often organized in Terahertz frequency modulation Step Frequency train of impulses, obtain the pulse signal after frequency displacement;

3b) pulse signal after frequency displacement is synthesized a large bandwidth signal according to the following formula:

$X\left(t\right)=\underset{m=1}{\overset{M}{\mathrm{\Σ}}}{y}_m\left(t\right)$

Wherein, X (t) represents the large bandwidth signal after synthesis, t represents the time that radar receiver reception chirp experiences, M represents the total number of chirp often organized Terahertz frequency modulation Step Frequency train of impulses and comprise, M value be more than or equal to 1 positive integer, m represents m chirp, and m span is the positive integer of 1 ~ M, y _mt () represents the pulse signal after the signal frequency shift after m pulse compression;

(4) windowing correction:

According to the following formula, to the large bandwidth signal windowing correction after synthesis, the revised large bandwidth signal of windowing is obtained:

Z(t)＝IFFT[FFT[X(t)]×H(f)]

Wherein, Z (t) represents the revised large bandwidth signal of windowing, t represents the time that radar receiver reception chirp experiences, IFFT [] expression carries out inverse Fourier transform to the large bandwidth signal of windowing correction, FFT [] represents that the large bandwidth signal after to synthesis carries out Fourier transform, X (t) represents the large bandwidth signal after synthesis

H (f) represents the rectangular window of frequency domain;

(5) RANGE-DOPPLER IMAGING:

5a) to the orientation of the revised large bandwidth signal of windowing to carrying out Fourier transform, obtain Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture;

5b) to the distance of the revised large bandwidth signal of windowing to carrying out pulse compression, obtain Terahertz inverse synthetic aperture radar (ISAR) target scattering point distance to full resolution pricture;

(6) image is obtained:

By Terahertz inverse synthetic aperture radar (ISAR) target scattering point orientation to full resolution pricture and distance to full resolution pricture comprehensively in two-dimensional coordinate system, obtain two-dimentional high-resolution Terahertz inverse synthetic aperture radar imaging figure.

2. the Terahertz inverse synthetic aperture radar imaging method based on frequency modulation Step Frequency according to claim 1, is characterized in that, step 3a) described in frequency displacement realize according to the following formula:

$y_m\left(t\right)=s_m\left(t\right)e^{j2\mathrm{\π\Δ}{f}_{m}t}$

Wherein, y _mt () represents the pulse signal after m frequency displacement, m represents m chirp, m span is the positive integer of 1 ~ M, M represents the total number of chirp often organized Terahertz frequency modulation Step Frequency train of impulses and comprise, M value be more than or equal to 1 positive integer, t represents that radar receiver receives time of experiencing of chirp; s _mt () represents the signal after m pulse compression, Δ f _mrepresent the frequency shift amount of the signal after m pulse compression.